A thread such as on a screw or bolt has a shape and design that characterizes its contour in space, this contour is called thread form. Traditionally a machine tool operator or inspector checks a thread form by using mechanical contact gauges or an optical comparator. These labor intensive methods are not very repeatable and yield only minimal quantitative information about the thread form. The techniques to make thread form measurements require an individual that is highly trained.
In view of the special methods and means of measurement which the general design of screw threads involve, compounded by the large variety of parameters which may be specified as important and assigned different tolerance grades, the measurement of screw threads has developed into a distinct branch of dimensional measurement.
The prior art employed three broad categories of devices for measuring characteristics of thread form: mechanical gauges, optical comparators and video analyzers. Prior art methods are used for measurements that both indicate acceptance or rejection of a part or to gain some quantitative information about a part. The acceptance or rejection of a part is generally known as "accept/reject measurements" termed "inspection by attributes". The gaining of quantitative information about a part is termed "inspection by variables".
Different mechanical gauges are required for different thread types and sizes. Consequently manufacturer's employing the methods of the prior art must maintain an inventory of gauges and purchase new gauges as new thread forms are manufactured. Mechanical gauges are also subject to wear and, consequently they need to be replaced periodically.
Specific examples of thread form measurement devices of the prior art are listed and described below: